This invention relates to an improved control circuit for use with a rotating eddy current probe wherein a characteristic of the probe can be varied while it is rotating.
In a retirement-for-cause (RFC) system wherein components, such as aircraft engine parts, are inspected individually for flaws and wear, and accepted or rejected on the basis of that inspection rather than being merely replaced automatically after some predetermined amount of use, regardless of actual condition, uses many different sizes of sensing devices, such as rotating eddy current probes. The two basic types of eddy current probes used are surface probes and hole probes.
A typical RFC inspection system uses many different sizes and configurations of rotating eddy current inspection probes since each surface characteristic requires a probe of the proper configuration because the clearance between the probe's coil and the part surface must be kept to a minimum in order to achieve the necessary sensitivity. For example, a hole inspection probe must be near the same diameter as the hole to be inspected, being only enough smaller to fit into the hole without making any contact with the part.
For each size probe there also must be an accompanying reference standard. Because of the very tight tolerances and difficulty in making the probes and reference standards, this prior art arrangement therefore becomes very expensive.
In addition, each time a different probe is used, there is a substantial amount of time involved in calibrating that probe to the reference standard. Calibration time for each probe is typically about 5 to 7 minutes.